Catalytic method of producing mercaptans from thioethers

ABSTRACT

The process for preparing a mercaptan from a thioether and hydrogen sulphide is carried out in the presence of hydrogen and a catalyst composition comprising a strong acid, such as a heteropolyacid, and at least one metal belonging to group VIII of the Periodic Table.

The present invention pertains to the field of mercaptans (also calledthiols) and relates more particularly to a catalytic process forpreparing mercaptans from thioethers and hydrogen sulphide in thepresence of hydrogen and a specific catalyst.

The industrial significance of mercaptans or thiols means that manystudies have been carried out for the purpose of perfecting thepreparation of these compounds. In particular a process is known whichis widely employed and which implements the reaction of hydrogensulphide with an alcohol or an olefin. In such a reaction a by-productwhich is obtained in particular comprises one or more thioethers, whichresult from secondary reactions and, primarily, from the reaction of themercaptan (formed in the main reaction) with the starting reactant, inother words either the alcohol or the olefin, depending on the processused.

Thioethers obtained as by-products during the preparation of mercaptansare not generally of commercial significance.

Methods of converting these thioethers for the purpose of upgrading themhave been proposed, the aim of these methods being to transform thethioethers into mercaptans by reaction with hydrogen sulphide (H₂S) inthe presence of various catalysts, in a reaction known assulfhydrolysis.

Existing sulfhydrolysis processes employ this reaction under pressure,using a reaction stream composed exclusively of H₂S and thioether invarious proportions, in the presence of various catalyst systems.

Thus U.S. Pat. No. 4,005,149 describes the preparation of mercaptans (orthiols) by reacting H₂S with organic sulphides (another name forthioethers) in the presence, as catalyst, of a sulphide of a metal fromgroup VI and/or of a metal from group VIII, particularly a sulphide ofcobalt and molybdenum (Co/Mo) impregnated on an alumina support. Carbondisulphide, CS₂, is added to the reaction mixture in order to improvethe conversion of the organic sulphide to mercaptan.

U.S. Pat. No. 4,396,778 describes a vapour-phase process for preparinghigh molecular weight C₁-C₁₈ alkyl mercaptans using as catalyst alarge-pore zeolite modified with potassium or sodium. The reaction iscarried out at a high temperature, greater than 290° C.

U.S. Pat. Nos. 2,829,171 and 3,081,353 describe the synthesis of lightermercaptans such as methyl mercaptan in the presence of activated aluminaas catalyst. The reaction temperatures employed in these processes arehigh.

Highly acidic ion exchange resins as described in U.S. Pat. No.4,927,972 are catalysts which are also employed in thioethersulfhydrolysis processes, but they generally lead to a low yield.

U.S. Pat. No. 4,059,636 describes the use of a solid catalyst comprisinga 12-phosphotungstic acid supported on alumina. This catalyst, comparedwith a customary catalyst such as molybdenum and cobalt supported onalumina (CoMo/Al₂O₃), has the effect of higher conversion and higherselectivity when it is employed in the sulfhydrolysis reaction, andachieves this with a lower reaction temperature. It may, however,require the presence of carbon sulphide, CS₂, as promoter. No indicationis given regarding the stability over time of this catalyst system.

A solid catalyst comprising a 12-phosphotungstic acid supported onsilica is also described by U.S. Pat. No. 5,420,092. That documentteaches, more generally, the use of a heteropolyacid in combination witha metal from group VIII, but in the distant field of the isomerizationof paraffins.

A new catalytic process has now been found for preparing a mercaptansfrom thioethers and hydrogen sulphide, which employs hydrogen in thereaction stream and a specific catalyst. It has the advantage ofutilizing lower temperatures, of obtaining high-purity mercaptans with agood yield, and of maintaining the high activity of the catalyst overtime.

The invention accordingly provides a process for preparing a mercaptanfrom a thioether and hydrogen sulphide, characterized in that it iscarried out in the presence of hydrogen and a catalyst compositioncomprising a strong acid and at least one metal belonging to group VIIIof the Periodic Table.

The combination of the hydrogen with this catalyst composition allowsthe activity of the catalyst to be stabilized at a high level over timeand at a relatively low temperature. This result is all the moresurprising for being obtained in a sulphurizing medium, which is knownto poison the active sites of catalysts.

The strong acid which can be used in the catalyst composition isselected from the group consisting of:

-   -   (a) one or more heteropolyacids selected from:        -   (i) a compound of formula: H₃PW₁₂O₄₀.nH₂O, H₄SiW₁₂O₄₀.nH₂O            or H₆P₂W₁₈O₆₂.nH₂O, in which n is an integer representing            the number of molecules of water of crystallization, and            (for a commercial product) is generally between 0 and 30,            preferably between 6 and 20;        -   (ii) a potassium, rubidium, caesium or ammonium salt of at            least one compound (i), or a mixture of such salts;    -   (b) a sulphated zirconium oxide,    -   (c) a tungstic zirconium oxide,    -   (d) a zeolite, and    -   (e) a cationic resin.

The heteropolyacid (i) is generally obtained by condensing two or moredifferent oxo acids, such as phosphoric acid, silicic acid or tungsticacid. It is soluble in water or in a polar organic solvent. The compoundof formula H₃PW₁₂O₄₀.nH₂O is known under the name of 12-phosphotungsticor 12-tungstophosphoric acid and is available commercially. The compoundof formula H₄SiW₁₂O₄₀.nH₂O is known under the name of 12-tungstosilicicor 12-silicotungstic acid, and is likewise available commercially. Thecompound of formula H₆P₂W₁₈O₆₂.nH₂O can be prepared according to theprocedure described in the following reference: A. P. Ginsberg,Inorganic Synthesis, Vol. 27, published by J. Wiley & Sons (1990) pages105-107.

The heteropolyacid (ii) is a salt obtained by partial substitution ofone or more protons of the heteropolyacid (i) by the correspondingcation. It is evident to the skilled person that such substitutioncannot be total without the acidity being lost. A salt of this kind isprepared from a solution of the heteropolyacid (i), to which the desiredamount of the alkali metal or ammonium precursor is added. The preferredprecursor is the corresponding chloride or carbonate. The precipitatedsalt is separated off and then dried under gentle conditions, preferablyby centrifugation followed by lyophilization. One reference which may bementioned is the following: N. Essayem, G. Coudurier, M. Fournier, J. C.Vedrine, Catal. Lett., 34 (1995) pages 224-225.

The sulphated zirconium oxide (b) is prepared by impregnating sulphuricacid on a zirconium oxide support in accordance with the processdescribed in the following reference:

F. R. Chen, G. Coudurier, J-F Joly and J. C. Vedrine, J. Catal., 143(1993) page 617.

The tungstic zirconium oxide (c) is prepared by impregnating tungstenoxide on a zirconium oxide support, in accordance with the processdescribed in U.S. Pat. No. 5,113,034 to Soled et al.

According to a first embodiment of the process according to theinvention the catalyst employed in the said process comprises as strongacid a heteropolyacid (ii), or one of the compounds (b), (c), (d) or(e). This version is preferred because, owing to the specific surfaceproperties of a strong acid of this kind, it is generally suitable as asupport. It is therefore not necessary in this case to deposit thestrong acid on a support.

The catalyst composition comprises in this case:

from 90% to 99.9%, preferably from 98.5% to 99.5%, by weight of strongacid, and

from 0.01% to 10%, preferably from 0.05% to 1.5%, by weight of metalfrom group VIII.

According to a second embodiment the catalyst employed comprises asstrong acid a heteropolyacid (i). This version is preferred owing to theparticularly advantageous activity of the catalyst in the sulfhydrolysisreaction.

The catalyst composition comprises in this case:

from 10% to 60%, preferably from 25 to 50%, by weight of strong acid,

from 0.01% to 10%, preferably from 0.1% to 2%, by weight of metal fromgroup VIII, and

from 30% to 80%, preferably from 48% to 75%, by weight of a supportselected from silica SiO₂, alumina Al₂O₃, titanium dioxide TiO₂,zirconium oxide ZrO₂, and activated carbon.

According to one particularly preferred embodiment the strong acidemployed in the catalyst is 12-phosphotungstic acid, preferablyimpregnated on silica.

The metal or metals belonging to group VIII of the Periodic Table thatis or are generally included in the catalyst composition employed is orare selected from, in particular, iron, cobalt, nickel, ruthenium,rhodium, palladium, osmium, iridium, and platinum.

Preference is given to employing a metal from group VIII that isselected from palladium, ruthenium and platinum, and is especiallypalladium.

One particularly preferred catalyst composition is that comprisingapproximately 40% by weight of 12-phosphotungstic acid, 1% of palladiumand 59% of silica.

The catalyst composition employed in a process according to theinvention may be prepared generally as follows:

When the strong acid used is one of the compounds (i):

(1) the support is heat-treated under vacuum at a temperature of between90 and 150° C., preferably of around 100° C., and then

(2) the support thus treated is impregnated with an aqueous or organicsolution of acid pH, containing the compound (i) and an acidic precursorof the metal from group VIII, and then

(3) the solid thus obtained is dried, and then

(4) is treated with H₂ at a temperature of between 80 and 300° C.,preferably between 180 and 250° C.

The aim of the heat treatment of step (1) is to desorb the water whichmay have been adsorbed in the pores of the support.

In step (2) the acidic precursor refers to a compound which in aqueoussolution gives rise to a cationic or anionic complex of the said metal.Examples of such compounds, in the case of platinum, are as follows:tetraammineplatinum hydroxide, tetraammine platinum chloride,dinitrodiamine-platinum(II), or else, in the case of palladium:palladium chloride, Pd(NH₃)₄Cl₂, (NH₄)₂(PdCl₄). Examples of suchcompounds further include, in the case of platinum: hexachloroplatinicacid (also called hydrogen hexachloroplatinate(IV)), ammoniumtetrachloroplatinate(II), and ammonium hexachloroplatinate(IV). The listof acidic precursors is given above purely by way of illustration,without limiting the compounds which can be used as an acidic precursorby the skilled person.

In step (3) the drying may be carried out, for example, by heating theimpregnated support, where appropriate under vacuum, at a temperature ofgenerally between ambient temperature and 120° C. for a time rangingfrom 30 minutes to 5 hours.

The H₂ treatment of step (4) is advantageously carried out on thecatalyst when the latter has been placed in the sulfhydrolysis reactor,and its purpose is to reduce the acidic precursor to metal from groupVIII.

When the catalyst employed comprises as strong acid a heteropolyacid(ii), or one of the compounds (b), (c), (d) or (e), it may be preparedby the same process except for the fact that the heat treatment is notmandatory, and must even be suppressed or modified, depending on thecharacteristics of the support.

The catalyst composition described above is employed in the process forpreparing mercaptan according to the invention, which comprises reactinghydrogen sulphide (H₂S) with a thioether in the presence of hydrogen.

This process is carried out in the gas phase, insofar as the temperatureand pressure conditions utilized are such that the reactants and theproducts are in the gaseous state.

The hydrogen is introduced into the process in an amount correspondingto a molar H₂S/H₂ ratio of between 10 and 200, preferably between 50 and100.

The thioether (or organic sulphide) used as starting reactant has thegeneral formula:R—S—R′  (I)

in which R and R′, which are identical or different, represent an alkylradical of 1 to 20 carbon atoms, preferably 1 to 12 carbon atoms, whichis linear or branched, or else a cycloalkyl radical of 3 to 7 carbonatoms.

Preference is given to using as starting thioether a compound of formula(I) in which R and R′ are identical. This is because, in this case,there is no need to separate the thiols obtained.

The thioether more preferably used is diethyl sulphide (or ethylthioether). The sulfhydrolysis reaction leads in this case to ethylmercaptan (or ethanethiol).

The hydrogen sulphide is introduced into the process in an amountsufficient to produce the conversion of the organic sulphide. Generallyspeaking, this amount corresponds to a molar H₂S/thioether ratio ofbetween 1 and 40, preferably between 2 and 30, more preferably between 2and 10.

The reactants described above are contacted in the presence of a chargeof the catalyst composition defined above in an appropriate reactionzone under reaction conditions appropriate for producing the desiredthiol.

The process is preferably implemented in a reactor which is fedcontinuously with the reactants, although a batch reactor may also beused.

The reaction temperature varies according to the thioether used and thedesired degree of conversion, but is generally situated within a rangeof between 50 and 350° C., preferably between 150 and 250° C.

The pressure at which the reaction is carried out also varies withinwide limits. Commonly it is situated at between atmospheric pressure and20 bars, preferably between 10 and 15 bars.

The contact time is generally between 1 and 50 s, preferably between 10and 30 s.

The thioether employed in the process according to the invention may bethe by-product obtained in a process for preparing thiol by addinghydrogen sulphide onto an alcohol or onto an olefin, in the presence ofa catalyst and/or by photochemical activation. In this process versionit is possible as a result advantageously to upgrade the saidby-product.

The examples below are given purely by way of illustration of theinvention, and must in no way be interpreted as constituting anylimitation thereon. In these examples the abbreviation HPW correspondsto the 12-phosphotungstic acid of formula H₃PW₁₂O₄₀.nH₂O.

EXAMPLE 1 Preparation of the Pd catalyst and HPW, supported on SiO₂

For 200 g of SiO₂, an aqueous solution is prepared which contains 6 g ofPdCl₂ and 140 g of HPW (weight expressed in equivalents of anhydrousacid, i.e. with n equal to 0).

The catalyst support used is an amorphous silica having a specific (orBET) surface area of 315 m²·g⁻¹, a pore diameter of the order of 12 to14 nm and a pore volume of 1.6 cm³·g⁻¹. This support is treated undervacuum beforehand at a temperature of 100° C.

The solution obtained above is impregnated onto the support thus treatedunder vacuum by aspiration. When impregnation of the solution has beencarried out, the mixture is stirred at atmospheric pressure for 1 hour.

The product obtained is dried under vacuum at ambient temperature and isthen subjected to treatment with hydrogen at a temperature of 230° C.for the purpose of reducing the palladium.

The catalyst obtained is composed of 59% by weight of SiO₂, 1% by weightof Pd and 40% by weight of HPW.

EXAMPLE 2 Preparation of ethyl mercaptan (CH₃CH₂—SH) from diethylsulphide (CH₃CH₂—S—CH₂CH₃):

A tubular reactor with a diameter of 25 mm is used which has a usefulcapacity of 200 ml and is charged with 200 ml of the catalystcomposition prepared according to example 1.

Passed through this charge per hour are 120 g of diethyl sulphide (or 1mol), 210 g of H_(s)S (or 5 mol) and 0.8 g of H₂ (or 0.08 mol).

The pressure in the reactor is maintained at 15 bars and the temperatureis set at 235° C.

Continuous analysis of the crude reaction products shows that theinitial conversion of the thioether is 52% , with an ethyl mercaptanyield of 49.3%.

EXAMPLE 3 Preparation of ethyl mercaptan (CH₃CH₂—SH) from diethylsulphide (CH₃CH₂—S—CH₂CH₃)— change in the conversion of ethyl mercaptanover time:

Example 2 is repeated, continuing the sulfhydrolysis reaction for 6 dayswith the same charge of catalyst composition, and periodically (as afunction of the time, expressed in days), measuring the conversion ofdiethyl sulphide (DES).

The results are collated in the table below. TABLE 1 Time (days)Conversion of DES (in %) 1 54 3 56 4 55 5 56 6 57

Table 1 shows that the catalyst system prepared in example 1 and used inthe presence of hydrogen according to the process of the inventionpossesses good stability over time.

1. Process for preparing a mercaptan comprising contacting a thioetherand hydrogen sulphide, in the presence of hydrogen and a catalystcomposition comprising a strong acid and at least one metal selectedfrom group VIII of the Periodic Table.
 2. Process according to claim 1,wherein the strong, acid is selected from the group consisting of: (a)one or mole heteropolyacids selected from the group H₃PW₁₂O₄₀.nH₂O,H₄SiW₁₂O₄₀.nH₂O or H₆P₂W₁₈O₆₂.nH₂O, in which n is an integerrepresenting the number of molecules of water of crystallization, and isbetween 0 and 30, potassium, rubidium, caesium or ammonium salts thereofand mixtures of such salts; (b) a sulphated zirconium oxide, (c) atulngstic zirconium oxide, (d) a zeolite, and (e) a cationic resin. 3.Process according to 1, wherein the strong acid is selected from thegroup potassium, rubidium, caesium or ammonium salts or a mixture ofsuch salts of H₃PW₁₂O₄₀.nH₂O, H₄SiW₁₂O₄₀.nH₂O or H₆P₂W₁₈O₆₂.nH₂O, inwhich n is an integer representing the number of molecules of water ofcrystallization, and is between 0 and 30, a sulphated zirconium oxide, atungstic zirconium oxide, a zeolite, and a cationic resin.
 4. Processaccording to claim 1, wherein the catalyst composition comprises: from90% to 99.9%, by weight of strong acid, and from 0.01% to 10%, by weightof at least one metal from group VIII.
 5. Process according to claim 1,wherein the strong acid is a heteropolyacid selected from the groupH₃PW₁₂O₄₀.nH₂O, H₄SiW₁₂O₄₀.nH₂O or H₆P₂W₁₈O₆₂.nH₂O, in which n is aninteger representing the number of molecules of water ofcrystallization, and is between 0 and
 30. 6. Process according to claim5, wherein the catalyst composition comprises: from 10% to 60%, byweight of strong acid, from 0.01% to 10%, by weight of at least onemetal from group VIII, and from 30% to 80%, by weight of a supportselected fibo silica SiO₂, alumina Al₂O₃, titanium dioxide TiO₂,zirconium oxide ZrO₂, and activated carbon.
 7. Process according toclaim 6, wherein the strong acid is 12-phosphotungstic acid.
 8. Processaccording to one of to claim 1, wherein the at least one metal isselected from iron, cobalt, nickel, ruthenium, rhodium, palladium,osmium, iridium, and platinum.
 9. Process according to claim 1, whereinthe at least one metal is selected from palladium, ruthenium, andplatinum.
 10. Process according to claim 1, wherein the at least onemetal is palladium.
 11. Process according to claim 1 wherein thecatalyst composition comprises approximately 40% by weight of12-phosphotungstic acid, 1% of palladium and 59% of silica.
 12. Processaccording to claim 1, wherein the hydrogen is introduced in an amountcorresponding to a molar H₂S/H₂ ratio of between 10 and
 200. 13. Processaccording to claim 1, wherein the thioether has the general formula:R—S—R′  (I) in which R and R′, which are identical or different,represent a linear or branched alkyl radical of 1 to 20 carbon atoms, orelse a cycloalkyl radical of 3 to 7 carbon atoms.
 14. Process accordingto claim 1, wherein the hydrogen sulphide is introduced in an amountcorresponding to a molar H₂S/thioether ratio of between 1 and
 40. 15.Process according to claim 1, wherein the catalyst compositioncomprises: from 98.5% to 99.9%, by weight of strong acid, and from 0.05%to 1.5%, by weight of at least one metal from group VIII.
 16. Processaccording to claim 5, wherein the catalyst composition comprises: from25 to 50%, by weight of strong acid, from 0.1% to 2%, by weight of atleast one metal from group VIII, and from 48% to 75%, by weight of asupport selected from silica SiO₂, alumina Al₂O₃, titanium dioxide TiO₂,zirconium oxide ZrO₂, and activated carbon.
 17. Process according toclaim 1, wherein the hydrogen is introduced in an amount correspondingto a molar H₂S/H₂ ratio of between 50 and
 100. 18. Process according toclaim 1, wherein the hydrogen sulphide is introduced in an amountcorresponding to a molar H₂S/thioether ratio of between 2 and
 30. 19.Process according to claim 1, wherein the hydrogen sulphide isintroduced in an amount corresponding to a molar H₂S/thioether ratio ofbetween 2 and
 10. 20. Process according to claim 1, wherein n is between6 and
 20. 21. Process according to claim 7, wherein said12-phosphotungstic acid is impregnated on silica.
 22. Process accordingto claim 13, wherein said linear or branched alkyl radical has 1 to 12carbon atoms.